The movable wall means in conventional tandem servomotors are normally constructed of two parts, namely a metal diaphragm plate and a rubber diaphragm, which are joined together by a series of bolts or interlocking threads on the mated parts. This type of arrangement is utilized because such parts are easily stamped or molded. In an effort to reduce the overall cost of such servomotors a uniform size hub for holding an operational control valve, as described in U.S. patent application Ser. No. 184,773, filed Sept. 29, 1971, and now U.S. Pat. No. 3,754,450 was developed to accommodate variable sized backing plates. The backing plate used in this servomotor is resiliently held against the hub by the diaphragm.